The camera’s “hack” allows the Solar Orbiter to peer deeper into the sun’s atmosphere

Scientists used the Solar Orbiter’s EUI camera in a new mode of operation to record a portion of the sun’s atmosphere at extreme ultraviolet wavelengths that until now have been nearly impossible to image. This new mode of operation was made possible by a last-minute camera “hack” and is sure to influence new solar instruments for future missions.

Solar Orbiter’s Extreme Ultraviolet Imager (EUI) returns high-resolution images of structures in the sun’s atmosphere. Scientists call this region the corona. During the construction of the EUI, a last-minute modification allowed the safety door on the front of the device to see through the target area deeper than originally intended.

“It was a real breakthrough,” says Frédéric Aucher, from the Institute of Astrophysics at Paris-Sud University and a member of the EUI team. “I came up with the idea to do that and see if it worked. It’s actually a very simple modification to the tool.”

It involved adding a small protruding “thumb” weighing a few grams to the instrument’s door. When the door slides away to let light into the camera, if it’s stopped halfway, the thumb covers the sun’s bright disk, and EUI can detect light a million times dimmer in ultraviolet coming from the surrounding corona.

The team refers to this as the mystery mode of operation. Tests with the EUI occulter have been running since 2021. Now the team is confident it will work and has written a paper in Astronomy and astrophysics It posted a video showing the results.

The film shows an ultraviolet image of the sun’s corona taken using an EUI cap. An ultraviolet image of the sun’s disk is superimposed in the centre, in the area left blank by the blocking. The image of the sun’s disk was taken by NASA’s STEREO mission, which happened to be looking at the sun from roughly the same direction as the Solar Orbiter at the same time, so features on the surface have a good correlation with features in the corona.

In the past, images of the sun’s corona were taken with specialized instruments called coronagraphs. For example, the solar orbiter’s coronagraph is called Metis. The value of this new approach is that the chronograph and camera can be included in the same instrument.

“We’ve shown that this works so well that you can now think of a new type of instrument that can image the sun and its corona,” says Daniel Müller, ESA’s Solar Orbiter project scientist.

And even before these new tools, there is a lot of new science that will come from the EUI. The mysterious mode allows scientists to see deeper into the sun’s atmosphere. This is the area that is outside the field of view of classical UV imagers but is usually obscured by conventional corona imagers. However, the EUI masking tool can now easily visualize this little-explored area.

“The physics is changing there, the magnetic structures are changing there, we haven’t looked at it so closely before,” says David Berghmans, Royal Observatory of Belgium and Astronomical Observatory. “There must be some secrets that we can find now.” EUI Principal Investigator.